As compared to a conventional buck converter, a multi-level buck converter has several advantages such as increased efficiency at high load states. In addition, the additional switches in multi-level buck converters in combination with the flying capacitor voltage being one half the input voltage lower the switching stresses as compared to conventional buck converters. Moreover, the ripple is reduced as the four switches in multi-level buck converters produce twice the ripple frequency as compared to the ripple frequency at the same switching speed for a conventional (single-phase) buck converter such that the switching frequency is effectively doubled for a multi-level buck converter. This increase in output ripple frequency advantageously enables a multi-level buck converter to use a smaller inductor as compared to a conventional buck converter.
Although multi-level buck converters thus offer advantageous properties over conventional buck converters, the control of the multiple switches for a multi-level buck converter is problematic. In general, there are four switching states for a four-switch multi-level buck converter as shown in FIG. 1. In each switching state, only two switch transistors are on from a set of four switch transistors: a switch transistor A, a switch transistor B, a switch transistor C, and a switch transistor D. In a switching state D1, switch transistors A and C are on such that the flying capacitor voltage VCF is charged by the input voltage and drives the switch node voltage VSW at the input of the inductor. The resulting inductor current ISW charges an output capacitor with the output voltage VOUT. In a switching state DV, switch transistors C and D are on such that the inductor freewheels and discharges into the output capacitor. The flying capacitor floats during switching state DV. In a switching state D2, switch transistors D and B are on such that the flying capacitor discharges into the switch node. Finally, switch transistors A and B are on in a switching state DP such that the switch node is charged to the input voltage VIN. The flying capacitor floats during switching state DP.
As compared to a conventional buck converter, the root-mean-square (RMS) switching node voltage VSW at the input node to the inductor is reduced by 50%. In particular, it can be shown that VSW will switch between the input voltage VIN and one-half of the input voltage if the output voltage is greater than one-half of the input voltage. Conversely, VSW will switch between VIN/2 and ground if VIN/2 is greater than VOUT. This reduction in the switch node voltage swing also reduces the switching voltage stresses on the switching transistors. Given the reduced voltage stress, the breakdown voltage ratings for the switching transistors may be reduced as compared to conventional buck converter switch transistors. Multi-level buck converters thus offer reduced conduction losses for its switch transistors.
But these advantages come at the cost of increased regulation complexity as plainly shown by the four switching states discussed above with regard to FIG. 1. Despite this increased complexity, prior-art multi-level buck converters have typically employed conventional buck converter control techniques such as valley-mode or peak-mode (peak-current) control. But the transition between valley-mode and peak-mode control in a multi-level buck converter creates a number of control stability issues that are not present in standard buck converters. In particular, note that a transition from peak to valley-mode control is typically unnecessary in a conventional buck converter over a wide range of operating conditions. But conventional multi-level buck converters that use current-mode control to maintain an amps-seconds balance on the flying capacitor transition between valley-mode and peak-current control when the duty cycle ranges from less than 50% to greater than 50% (the duty cycle being defined as the ratio of the output voltage to the input voltage). It is thus conventional to limit multi-level buck converter control to just one of the valley-mode and peak-current control modes. But such a control limitation in turn limits the duty cycle range. There is thus a need in the art for improved multi-level buck converter having a regulation over a wide input voltage range.
The restriction in operating range is not the only issue facing conventional multi-level buck converters. In addition, multi-level buck converters suffer from non-ideal flying capacitor voltage levels. Given its topology, the flying capacitor voltage will ideally average to VIN/2. Similarly, the switch node voltage will average to VIN/2 for switching states D1 and D2. In contrast, the switch node voltage is grounded in switching state DV and equals VIN in switching state DP. Given these three possible voltage values, a multi-level buck converter such as illustrated in FIG. 1 may also be denoted as a three-level buck converter. The sum of the D1 and D2 switching state periods times the (ideal) flying capacitor voltage VIN/2 equals the output voltage. As noted earlier, the ratio of the output voltage to the input voltage for a multi-level buck converter may be deemed to define a duty cycle ratio D such that D=VOUT/VIN. Assuming that flying capacitor voltage VCF is one half of the input voltage, the duty cycle D thus equals one half the sum of the D1 and D2 switching periods. Under ideal conditions, the flying capacitor voltage will self-regulate to VIN/2 but imbalances due to differences in parasitic elements such as the switch capacitance causes the flying capacitor voltage to drift towards ground or towards VIN. Either outcome significantly impairs the multi-level buck controller from regulating the output voltage. Moreover existing schemes to regulate the flying capacitor voltage complicate the output voltage regulation. Accordingly, there is a need in the art for improved multi-level buck converters that may be regulated over a wide VIN to VOUT ratio (a broad duty cycle range) while also regulating the flying capacitor voltage.
As compared to a conventional buck converter, driving the switch transistors in a multi-level buck converter presents additional challenges due to the problems of supplying the gate drive to switch on the three floating switch transistors. In contrast, a conventional buck converter has only one floating switch transistor. Both types of buck converter will have one switch transistor coupled to ground. It is conventional for a multi-level buck converter to be implemented with NMOS switch transistors due to their advantageously-low on resistances. But producing the gate drive voltages sufficient to switch on the floating NMOS switch transistors in a multi-level buck converter is problematic.
There is thus a need in the art for improved gate drivers for the switch transistors in a multi-level buck converter.